1. Field of the Invention
This invention relates to a control circuit for controlling a driving current of a current driver in a motor driving circuit, and more particularly to simplification of such a control circuit and improvement of its control characteristics.
2. Description of the Related Art
Heretofore motor drivers of full-wave drive type are known as motor drive circuits. The principles of this type of motor drivers will now be described with reference to FIG. 3 of the accompanying drawings.
In FIG. 3, the rotational position of a motor 10 is detected by a rotational position detector (not shown) such as a hole element. Assuming that a detection of the position is made for example by three hole elements, position detection signals H1, H2, H3 are obtained to be inputted to a drive signal generator 12. This drive signal generator 12 issues signals for changing over the respective current-flow directions of a plurality of coils of the motor depending on the position detection signals H1, H2, H3.
The output signals of the drive signal generator 12 are supplied to a current driver 16, via a driving current control circuit 14, to control a current in each of the coils (e.g., three coils in FIG. 3) of the motor.
FIG. 4 shows a current driver 16 of full-wave drive type, which controls the direction of the current flowing to the respective coil and which is composed of current-limiting elements corresponding one to each coil.
In this illustrated example, the individual current-limiting element comprises a p-n-p transistor 20 and an n-p-n transistor 22 which are interconnected in series. The emitter of the p-n-p transistor 20 is connected to a power source, and the emitter of the n-p-n transistor 22 is connected to the ground; and the junction of the collectors of these two transistors 20, 22 is connected to a respective one of the coils of the motor 10.
When the transistor 20 is rendered conductive, the driving current flows in the coil of the motor 10. When the transistor 22 is rendered conductive, the driving current flows in the coil of the motor 10.
This driving current may be controlled by:
1) controlling only the power-source-side transistor 20; PA1 2) controlling only the ground-side transistor 22; or PA1 3) controlling both the power-source-side transistor 20 and the ground-side transistor 22.
By any of these three ways, it is possible to supply a desired current to the coil of the motor 10.
The motor driving current flow in the current driver 16 is controlled by the driving current control circuit 14. One example of this driving current control circuit 14 will now be described with reference to FIGS. 5A and 5B.
The driving current control circuit 14 includes a driving current controlling circuit 14a for outflow of a base current, and a driving current controlling circuit 14b for inflow of a base current.
The driving current controlling circuit 14a of FIG. 5A is for outflow of the base current of the transistor 20 and is composed of two current mirrors 30, 32. The collectors of a pair of p-n-p transistors 34, 36 of the current mirror 30 are connected to the power source. And a current regulator 38 is interposed between the collector and the ground of the input-side transistor 34.
For instance, the current regulator 38 increases and reduces the current value according to a control signal from the exterior which signal corresponds to the torque command for the motor 10.
The collector of the output-side transistor 36 of the current mirror 30 is connected to the input-side transistor 42 of the current mirror 32 via a resistor 40. The emitter of this transistor 42 is connected to the ground. The emitter and the collector of the output-side transistor 44 of the current mirror 32 are connected to the ground and the terminal T.sub.1, respectively.
Consequently, partly since the same amount of current flows to the input-side and output-side transistors 34, 36 of the current mirror 30, and partly since the same amount of current flows to the input-side and output-side transistors 42, 44 of the current mirror 32, a current equal to the amount of current flowing to the current regulator 38 flows in from the terminal T.sub.1.
A phase compensating condenser 46 is connected to the junction of the collector of the transistor 36 and the resistor 40. With this phase compensating condenser 46, it is possible to delay the phase of the current flowing in from the terminal T.sub.1. In general, the driving current control circuit 14 is an IC form; the phase compensating condenser 46 is large in size, and is therefore attached to the IC via the terminal P.
By controlling the base current of the transistor 20 in the current driver 16 by the driving current controlling circuit 14a, it is possible to control the current flow to the individual coil of the motor 10.
The driving current controlling circuit 14b of FIG. 5B is for supply of the base current of the transistor 22 in the current driver 16. This driving current controlling circuit 14b is identical with the driving current controlling circuit 14a, except that + (plus) and - (minus) are just inverted. Namely, the driving current controlling circuit 14b, like the driving current controlling circuit 14a, is composed of two current mirrors 30, 32, a current regulator 38, a resistor 40 and a phase compensating condenser 46; the power source and the ground, to which these respective parts are connected, are arranged oppositely. Also the junction, i.e., p-n-p or n-p-n of the transistors 34, 36, 42, 44 are opposite. Therefore, from the terminal T.sub.2 the base current flows out to each transistor 22 in the current driver 16. Here the current flowing out from this terminal T.sub.2 is synchronous with the current flowing in from the terminal T.sub.1.
Since the supply of current in the current driver 16 to the motor 10 is controlled, in synchronism with the drive signal, by the driving current control circuit 14, a desired rate of rotation of the motor 10 can be achieved.
However, in the conventional art, the driving current controlling circuit 14a constituting the ground-side circuit of the current driver 16 for inflow of the base current is totally independent from the driving current controlling circuit 14b constituting the power-source-side circuit of the current driver 16 for supply of the base current. For this reason, a phase divergence would occur between the two controlling circuits 14a, 14b. Further, it is very difficult to control the respective currents while maintaining the ratio of the driving currents of the two controlling circuits 14a, 14b.
Furthermore, each of the two driving current controlling circuits 14a, 14b requires one phase compensating condenser 46, i.e., two condensers in total. A condenser is large in size and so is a part not suitable to be incorporated in an IC. Consequently it has been a common demand in the art that the number of the condensers could be minimized.